Radiation therapy involves medical procedures that selectively expose certain areas of a human body, such as cancerous tumors, to high doses of radiation. Radiation may also be used to obtain images of the patient during an imaging procedure.
The goal of radiation treatment delivery is to deliver the prescribed dose of the treatment plan as precise as possible, i.e., following the planned volumetric dose distributions to both the target (e.g., tumor) as well as the surroundings (e.g., organs at risk). This is commonly known as “conformity” and is one of the parameters in treatment planning.
During treatment delivery, this conformity is currently not systematically quantified. Instead, single axis deviation limits are evaluated, which specifies tolerances for the position of any moving axis, such as the gantry rotation or a leaf of a multi-leaf collimator. These single axis limits do not quantify deviation of several axes. In particular, they do not quantify deviation of several axes but still below the limit—e.g., a single multi-leaf collimator (MLC) leaf might stop the treatment by exceeding the tolerance, while the treatment would not have been stopped if all MLC leaves are offset by just less than the limit. Also, these single axis limits may not be clinically relevant. For example, a couch axis deviation in the direction along the treatment beam may not have significant impact on the treatment delivery since the difference in the depth dose resulting from such deviation would be marginal. However, the same couch axis deviation may have a larger dosimetric effect later in the same treatment if the gantry is rotated by 90 degrees.
Also, for dynamic tracking of target, there may be no conformity check in place. Instead, the single axis tolerances are widened up to disable the axis limits to thereby allow for aperture adaptation (i.e., modifying positions of the MLC leaves to adapt to the target movement, or moving a patient support to place the target in alignment with the aperture of the collimator where the beam is exiting therethrough). Tracking conformity is then quantified for the end-to-end system test using volumetric phantoms. However, quantification of tracking conformity using volumetric phantoms is limited to simulated motion patterns, and is not available during treatment of real patient. Also, the above technique of widening up single axis tolerances may be hazardous to the patient, and may hinder clinical implementation of dynamic tracking protocols.